A conventional strain sensor is disclosed in Japanese Utility Model Unexamined Publication No. H5-57605, and is described below with reference to drawings.
FIG. 8 is a perspective view of the conventional strain sensor, and FIG. 9 is a side sectional view illustrating the conventional strain sensor fixed to its detection target. Metal sensor substrate 1 has fixing holes 2 and 3. Strain-detecting element 4, consisting of a strain gauge, is disposed on the top face of sensor substrate 1. Strain-detecting element 4 includes element portion 5, and lead 6 electrically coupled to element portion 5 and protruding outward.
The operation of the conventional strain sensor as configured above is described next with reference to FIG. 9. A pair of internal threads 7 is provided in advance on detection target 8. Sensor substrate 1 is fixed to detection target 8 by screwing external threads 9 into internal threads 7 with sensor substrate 1 therebetween. The inner diameter of fixing holes 2 and 3 is slightly greater than the outer diameter of internal threads 7. This is to ensure the reliable fixing of sensor substrate 1 onto detection target 8, considering variations in the pitch of internal threads 7 provided on detection target 8. In this state, when an external force is applied to detection target 8, detection target 8 distorts, and sensor substrate 1 deforms in response to this distortion. The deformation of sensor substrate 1 is output from lead 6 as a change in voltage caused by a change in resistance of strain-detecting element 4 provided on the top face of sensor substrate 1. Accordingly, the external force applied to detection target 8 is detectable.
However, in the above conventional structure, a twisting force acts on external threads 9 when sensor substrate 1 is fixed to detection target 8 by screwing external threads 9 into internal threads 7. This twisting force produces internal stress in sensor substrate 1. As a result, strain-detecting element 4 distorts, and an output signal is generated even when no external force is applied to sensor substrate 1. Consequently, output signals from the strain sensor become inaccurate.